The ( 3 He,p) Reaction to Study T=1 and T=0 Pairing in N=Z Nuclei

Transcription

1 The ( 3 He,p) Reaction to Study T=1 and T=0 Pairing in N=Z Nuclei A. O. Macchiavelli Lawrence Berkeley National Laboratory Workshop on Pairing Degrees of Freedom in Nuclei and the Nuclear Medium INT - University of Washington November 14-17, 2005

2 Motivation N=Z nuclei, unique systems to study np correlations As you move out of N=Z nn and pp pairs are favored Role of isoscalar (T=0) and isovector (T=1) pairing Large spatial overlap of n and p Pairing vibrations (normal system) Pairing rotations (superfluid system) Does isoscalar pairing give rise to collective modes? What is (are) the smoking-gun(s)? Binding energy differences Ground states of odd-odd self-conjugate nuclei Rotational properties: moments of inertia, alignments Two-particle transfer cross-sections

3 The smoking gun?

4

5 Two nucleon transfer reactions Generalized densities a+a+, aa represent the pair field and in close analogy to the collective excitations corresponding to the ordinary density, they can give rise to collective modes. V vibrations rotations = G < Σa a > v v Two particle transfer reactions like (t,p) or (p,t), where 2 neutrons are deposited or picked up at the same point in space provide an specific tool to probe the amplitude of this collective motion. The transition operator <f a+a+ i> will be proportional to the pair density of the nucleus.

6 Collective pairing vibrations near closed shells l,ω D ΩG / D < 1 Collective excitations have a phonon-like spectrum h ω ( 1 ) 1 / 2 ~ D x x = G / G crit

7 Study binding energies around closed shells ( 56 Ni) T=0 Energy comparable with single particle separation - low collectivity. T=1 Energy consistent with collective excitations.

8 ( 3 He,p) Transfer Reactions σ? < 1 T 0 > 2 1/ hω T=0 J=0 Even-even np ( 3 He,p) T=0 J=1 T=1 J=0 Odd-odd L=0 transfer forward peaked Measure the np transfer cross section to T=1 and T=0 states Both absolute σ(t=0) and σ(t=1) and relative σ(t=0) / σ(t=1) tell us about the character and strength of the correlations

9 R.Chasman

10 40 Ca( 3 He,p) 42 Sc 200MeV L=0 transfer

11 Degrader 10mg/cm 2 ~10 5 /sec From ATLAS 56 Ni 58 Cu 3 He Au FMA Si detector 500µ 16x16 ~1sr Gas cell ~100µg/cm 2 20 counts/day

12 Proof of principle A.O.Macchiavelli 1, E.Rehm 2, A.Görgen 1, P.Fallon 1, M.Cromaz 1, C.N.Davis 2, A.Heinz 2, R.V.F.Janssens 2,C.L.Jiang 2, E.F.Moore 2, G.Mukherjee 2, R.Pardo 2, D.Seweryniak 2, J.P.Schiffer 2, J.Cizewski 3, J.Thomas 3, M.Paul 4 1 Nuclear Science Division, Lawrence Berkeley National Laboratory 2 Physics Division, Argonne National Laboratory 3 Department of Physics and Astronomy, Rutgers University 4 Hebrew University Monitor Si detector Gas Cell Beam Au degrader

13

14 ICdE1 K Cl P Sc PPAC SiE IC

15 FMA Gated Raw

16 dσ/dω (mb/sr) Sc 1 + T=0 Nucl.Phys. A80 (1966) Nucl.Phys. A116 (1968) θc.m. (deg)

17 What is our reference? Ratio(T=1/T=0) θc.m. (deg) Single-particle estimate ~ (spin)x( 3 He)x(LS -> jj)

18 Systematic of ( 3 He,p) and (t,p) reactions in stable N=Z nuclei??

19 Ratios using both (t,p) and (3he,p). The blue line is the sp estimate assuming that the j2 configuration varies from an s1/2 to a j>>1

20 Summary and Conclusions Ground State Binding Energies (pair gaps) Energies of T=0 T=1 in N=Z nuclei Excitation spectra near shell gaps (pair vibrations) Evidence for full isovector T=1 pairing (nn,np,pp) - charge independence. BE differences can be described by an appropriate combination of the symmetry energy and the isovector pairing energy. No evidence for a T=0 deuteron-like pairing condensate in N=Z nuclei. The T=0 states in an odd-odd N=Z nucleus can be characterized as a seniority 2 state (as in any other odd-odd nucleus). Inverse kinematics - Successful test with stable beams Next step - Measure collectivity with transfer reactions ( 3 He,p) Approved ATLAS runs with 44 Ti and 56 Ni beams Role of pairing phonons near 40 Ca and 56 Ni

21 And looking ahead